1. Field of the Invention
The present invention relates to a process and apparatus for a fluid catalytic cracking (FCC) unit. More particularly, the present invention relates to a process and apparatus for reducing residence time of stripped hydrocarbon vapors within an FCC reactor vessel (disengaging vessel) by employing a vented stripper section.
2. Discussion of the Prior Art
By way of background, the hydrocarbon conversion catalyst usually employed in a FCC installation is preferably a high activity crystalline zeolite catalyst of a fluidizable particle size. The catalyst is transferred in suspended or dispersed phase condition with a hydrocarbon feed generally upwardly through one or more riser conversion zones (FCC cracking zones) providing a hydrocarbon residence time in each conversion zone in the range of 0.5 to about 10 seconds and usually less than about 8 seconds. High temperature riser hydrocarbon conversions, occurring at temperatures of at least 980.degree. F. and a 0.5 to 4 seconds hydrocarbon residence time in contact with the catalyst in the riser, are desirable for some operations before initiating separation of vaporous hydrocarbon product materials from the catalysts.
Rapid separation of catalyst from hydrocarbons discharged from a riser conversion zone is particularly desirable for restricting hydrocarbon conversion time. During the hydrocarbon conversion, carbonaceous deposits accumulate on the catalyst particles and the particles entrain hydrocarbon vapors upon removal from the hydrocarbon conversion zone. The entrained hydrocarbons are subjected to further contact with the catalyst until they are removed from the catalyst by a separator, such as cyclonic equipment, or stripping gas in a separate catalyst stripping zone or both. Hydrocarbon conversion products separated and stripped from the catalyst are combined and typically passed to a product fractionation step. Stripped catalyst containing deactivating amounts of carbonaceous material, hereinafter referred to as coke, is then passed to a catalyst regeneration operation.
Of particular interest has been the development of methods and systems for efficiently separating catalyst particles from a gasiform mixture of catalyst particles and vaporous hydrocarbon product which is discharged from the riser. Such separation reduces overcracking of hydrocarbon conversion products and promotes the recovery of desired products of a hydrocarbon conversion operation.
Various process and mechanical means have been employed heretofore to effect rapid separation of the catalyst phase from the hydrocarbon phase, at the termination of the riser conversion zone, to minimize contact time of the catalyst with cracked hydrocarbons.
U.S Pat. No 4,502,947 to Haddad et al discloses a closed cyclone fluid catalytic cracking catalyst separation method and apparatus. In the closed cyclone method and apparatus, hydrocarbon product from a riser is separated from catalyst and discharged from a disengaging vessel (reactor vessel) without substantially passing into an atmosphere of the disengaging vessel. Preventing passage of the hydrocarbon into the atmosphere of the disengaging vessel reduces the time for overcracking the hydrocarbon product by high temperature thermal cracking and overly long contact with catalyst.
U.S. Pat. No. 4,572,780 to Owen et al discloses a method and apparatus for fluid catalytic cracking of a hydrocarbon feed and is incorporated herein by reference. The method and apparatus includes a multistage stripper system. The multistage stripper system comprises a means for spinning a gasiform mixture of catalyst and cracked hydrocarbons exiting from a riser, a first means for stripping the spun gasiform mixture, and a means for deflecting the gasiform mixture to separate catalyst from the cracked hydrocarbons. A downstream end of a riser conversion zone may terminate within a stripper vessel located within a disengaging vessel (reactor vessel).
A need exists for developing new systems, for decreasing residence time of hydrocarbon vapor products stripped away from catalyst in a fluid catalytic cracking reactor vessel. Particularly a need exists for systems useful in modifying existing designs. Short residence times for stripped hydrocarbon vapor is especially valuable as the technology progresses to employing higher reaction temperatures and heavier feedstocks.